1. Field of the Invention
This invention relates to instruments for measuring pressure and in particular to a cell for use in an instrument for measuring oncotic pressure of blood serum or plasma, or the like.
2. Prior Art
Instruments are known which have proved to be very useful diagnostic and measuring tools used for heart disease treatment, for example. They usually consist of an upper sample chamber into which a very small amount of the blood serum or plasma is placed and a lower chamber into which a reference solution is inserted. The lower reference chamber has an axial aperture and passageway which communicates with the upper sample chamber only through a semi-permeable membrane placed over the aperture. This membrane passes certain electrolytes and other constituents of the blood serum or plasma downward into the reference solution, but retains high molecular weight molecules such as proteins. Pressure exerted on the membrane by the passage of these constituents is sensed by a pressure transducer which is coupled to the lower end of the reference chamber. This pressure is quantified and displayed or recorded. Among other uses, the measurement of the oncotic pressure in heart disease helps to provide data indicating the risk of pulmonary edema and survival potential as well as aiding in prescribing the proper form of therapy.
One such prior art instrument is made almost entirely of plastic except for the pressure transducer coupled to the lower reference chamber. The upper sample chamber is threaded and is screwed onto the top of the reference chamber after the semi-permeable membrane disc has been placed between the chambers over the aperture in the reference chamber. While screwing the sample chamber onto the reference chamber exerted downward pressure upon a washer or ring placed above the disc, the screwing action often was sufficient to mutilate the disc. Thus, sometimes the operator would inadvertently destroy five or six discs before the semi-permeable membrane disc was held properly in place in an unmutilated state after the sample chamber was screwed on. Only if the disc is unmutilated can the disc effectively and perfectly form the right kind of barrier between the chambers. The necessity of having to produce this desired condition by repeated trials can result in delays which cannot be tolerated when the very life of the patient from whom the sample was drawn requires immediate and accurate diagnosis of his condition.